Dzhokhar Tsarnaev (pictured in a white hat) stands behind 8-year-old Martin Richard just before the second bomb exploded at 2013 Boston Marathon.

The last question lead prosecutor in the Boston Marathon bombing case and Assistant U.S. Attorney Bill Weinreb asked before resting the government's case: "How old was Martin Richard?"

"He was 8 years old," replied Boston's Chief Medical Examiner Dr. Henry Nields, who performed the autopsy on the youngest victim killed in the 2013 bombing.

The government concluded its presentation of evidence Monday after calling 92 witnesses over 15 days of testimony in the guilt phase of the trial.

The prosecution's final witness spent close to an hour describing the horrific destruction that was done to Richard's body as the child stood on metal barrier along the marathon course on Boylston Street in front of the Forum restaurant.

The scene in front of the Forum restaurant before and after the second bomb exploded.

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"Again, jurors these are graphic images," said Judge George O'Toole to prepare the jury, as the autopsy photos of Martin Richard were shown on their courtroom screens.

Close to half the jury was moved to tears as they viewed Richard's autopsy photos while listening to Nields describe Richard's injuries, which included a 6-by-6-inch portion of Richard's torso that was blown off, exposing organs and intestines.

The autopsy photos were only shown to the jury and not the courtroom spectators, including Richard's parents, Bill and Denise, who were present Monday, as they have been for most of the trial.

On the second day of the trial, Bill Richard gave harrowing testimony about how he was thrown into the street when the second bomb exploded in front of the Forum. Moments later, when he went to retrieve his kids, he found Martin lying on the sidewalk.

"I saw Denise and other people hovering over trying to help Martin. I knew that I needed to get back and help Jane. When I saw Martin's condition I knew that he wasn't going to make it," Richard said.

The defense for 21-year-old Boston bomber Dzhokhar Tsarnaev began its case Monday afternoon.

As the defense opened its case, Tsarnaev's attorneys filed a motion to dismiss or acquit him of all charges against him. The motion asserts that "the government has failed to introduce evidence sufficient to establish each essential element of the offenses charged beyond a reasonable doubt."

Tsarnaev has pled not guilty in the case, although his lawyer told the jury on the first day of the trial that her client was behind the crimes: "It was him," Judy Clarke said.